The subject matter herein relates generally dielectric waveguides.
Dielectric waveguides are used in communications applications to convey signals in the form of electromagnetic waves along a path. Dielectric waveguides provide communication transmission lines for connecting communication devices, such as connecting an antenna to a radio frequency transmitter and/or receiver. Although waves in open space propagate in all directions, dielectric waveguides generally confine the waves and direct the waves along a defined path, which allows the waveguides to transmit high frequency signals over relatively long distances.
Dielectric waveguides include at least one dielectric material, and typically have two or more dielectric materials. A dielectric is an electrical insulating material that can be polarized by an applied electrical field. The polarizability of a dielectric material is expressed by a value called the “dielectric constant” or “relative permittivity.” The dielectric constant of a given material is its dielectric permittivity expressed as a ratio relative to the permittivity of a vacuum, which is 1 by definition. A first dielectric material with a greater dielectric constant than a second dielectric material is able to store more electrical charge by means of polarization than the second dielectric material.
Some known dielectric waveguides include a core dielectric material and a cladding dielectric material that surrounds the core dielectric material. The dielectric constants, in addition to the dimensions and other parameters, of each of the core dielectric material and the cladding dielectric material affect how an electromagnetic field through the waveguide is distributed within the waveguide. In known dielectric waveguides, the electromagnetic field typically has a distribution that extends radially through the core dielectric material, the cladding dielectric material, and even partially outside of the cladding dielectric material (for example, within the air outside of the waveguide).
There are several issues associated with portions of the electromagnetic field extending outside of the cladding of the dielectric waveguide into the surrounding environment. First, the portions of the electromagnetic field outside of the waveguide may produce high crosstalk levels when multiple dielectric waveguides are bundled together in a cable, and the level of crosstalk increases with higher modulated frequencies propagating through the waveguides. Second, some electromagnetic fields in air may travel faster than fields that propagate within the waveguide, which leads to the undesired electrical effect called “dispersion.” Dispersion occurs when some frequency components of a signal travel at a different speed than other frequency components of the signal, resulting in inter-signal interference. Third, the dielectric waveguide may experience interference and signal degradation due to external physical influences that interact with the electromagnetic field, such as a human hand touching the dielectric waveguide. Finally, portions of the electromagnetic field outside of the waveguide may be lost along bends in the waveguide, as uncontained fields tend to radiate away in a straight line instead of following the contours of the waveguide.
One potential solution for at least some of these issues is to increase the overall diameter of the dielectric waveguides, such as by increasing the diameter of the cladding layer or the diameter of a dielectric outer jacket layer that surrounds the cladding layer. Increasing the amount of dielectric material provides better field containment and reduces the amount or extent of the electromagnetic field propagating outside of the waveguide. But, increasing the size of the dielectric waveguide introduces other drawbacks, including reduced flexibility of the waveguides, increased material costs, and a reduced number of waveguides that can fit within a given area or space (for example, reducing the density of waveguides).
Another potential solution is to provide an electrically conductive shielding layer that engages the outer dielectric layers of the waveguide and surrounds the full perimeter of the waveguide. But, completely encasing the dielectric waveguide with a conductive material can cause undesirably high energy loss levels (for example, insertion loss and/or return loss) in the waveguides as portions of the electromagnetic fields induce surface currents in the conductive material. High loss levels shorten the effective length that an electromagnetic wave will propagate through the waveguide. Furthermore, outer metal shielding layers interacting with the propagating electromagnetic waves can allow undesirable modes of propagation that have hard cutoff frequencies. For example, at some specific frequencies, the shielding layers can completely halt or “cutoff” the desired field propagation.
A need remains for a dielectric waveguide for propagating high frequency electromagnetic signals that has a relatively compact size and a reduced sensitivity to external influences (for example, crosstalk and other interference), while providing acceptably low levels of loss and avoiding unwanted mode propagation.